The use of sulfur-containing fuels such as coal results in flue gases contaminated with gaseous sulfur dioxides environmentally undesirable in that this oxide is thought to be one of the major sources of acid rain and therefore the permissible levels of emission of sulfur dioxides is regulated in order to provide that these vented gas streams enter the atmosphere with the minimal impact of introduction of miniscule quantities of sulfur-containing pollutant gases.
In order to meet these regulatory criteria, producers of the contaminant sulfur dioxide derived numerous flue gas scrubbing methods for desulfurization utilizing stoichiometric amounts of dry alkaline reagents such as lime, limestone, sodium carbonate, sodium carbonate in tandem with lime, magnesium oxide, sodium sulfite, spent shale, and basic aluminum sulfate as well as aqueous solutions and slurries of these aforementioned reagents to absorb the sulfurous gases and convert them to sulfite radicals with a usual subsequent oxidation to the sulfate radical followed by disposal or regeneration of the absorbing medium.
A particular method for achieving this absorption and subsequent oxidation of sulfur dioxide in present use is known as the DOWA process. This DOWA process, as described in U.S. Pat. No. 3,943,230, utilizes a buffered acid medium flue gas desulfurization method which consists of scrubbing said flue gas with an aqueous solution of basic aluminum sulfate, thereby transforming sulfur dioxide to the sulfite radical, oxidizing this aqueous solution in an oxidizing unit to the sulfate radical, and then reacting a small portion of this alum solution with calcium carbonate to precipitate calcium sulfate and to regenerate the basic aluminum sulfate solution so as to afford a recirculation of the absorbing medium.
The basic aluminum sulfate solution is described as having a certain "percentage of basicity" calculated as the amount (equivalents) of aluminum in solution as aluminum hydroxide, divided by the total aluminum content of the solution, and multiplied by 100. For illustration, if a one molar solution of aluminum sulfate has one mole of calcium carbonate added to it to form one mole of calcium sulfate, the resulting solution is described as having a 33.3% basicity since two of the six equivalents of aluminum in solution have been converted to the basic form by losing the sulfate radical and gaining two hydroxide radicals, ##EQU1##
A second method of control described in Japanese Pat. Nos. JP5700207530 and JP58008538 teaches the use of a water slurry of spent shale at high pH to absorb the sulfur dioxide from flue gases with the stoichiometric amounts of shale in large quantities of water.
In order to minimize the quantities of shale necessary and to minimize the quantity of water to be pumped through the flue gas scrubber, a process had to be devised in which the shale acted catalytically instead of stoichiometrically and in which a solution of basic aluminum sulfate was to be used instead of pure water since the said solution had greater capacity for absorbing the sulfur oxide gas. With such a process the absorption of sulfur oxide gas would be carried out on flue gas with a large decrease in the amount of materials to be handled as well as the unexpected result that an oxidizing unit was only necessary wherein the flue gas was devoid of oxygen since the shale catalyst functioned in the presence of the usual concentrations of oxygen in flue gas.